The present invention relates to a method for zinc phosphate surface chemical conversion treatment of metal products such as automotive bodies, household electrical appliances and steel furniture.
Metallic products such as automotive bodies, household electrical appliances, steel furniture, etc. are generally subjected to a zinc phosphate chemical conversion treatment prior to coating. While this treatment is generally carried out by a spray technique or a dip technique, dip chemical conversion followed by cationic electrocoating is the coating system generally applied to metallic substrates having an intricate surface structure and calling for a corrosion-resistant surface after coating as it is true of automotive bodies. Regarding the substrate as such, one having both an iron type surface and a zinc type surface is usually applied thereto.
The conventional process for zinc-phosphating metallic substrates comprises a sequence of degreasing-aqueous cleaning-aqueous cleaning-chemical conversion-aqueous cleaning-aqueous cleaning. In the chemical conversion stage, the treating agent is replenished to make up for its consumption due to the chemical conversion and carry-over loss of said agent so as to control the concentrations of zinc and other metal ions, total acidity, acid ratio and other process parameters at constant values. Furthermore, the NO2 concentration of the treating bath is maintained at a constant amount generally by feeding an aqueous solution of sodium nitrite as a chemical conversion accelerator. However, such a bath management procedure is not only uneconomical in that the sodium ion unnecessary for chemical conversion must be added but also disadvantageous in that the increase in sodium ion concentration elevates the pH of the treating bath to cause precipitation of chemical conversion film-forming components in the treating bath. Moreover, NO2 in the treating agent is oxidized to the nitrate ion to thereby increase the nitrate ion concentration of the treating agent.
Meanwhile, in the phosphating line in general use today, where a portion of the treating agent is carried over to the aqueous cleaning stage as mentioned above, the accumulation of sodium and nitrate ions beyond the necessary levels in the treating agent may be prevented and a balance of treating agent ion concentrations maintained by supplementing the treating agent at rates commensurate with consumption due to carry-overs. However, as the amount of carry-overs of any component of the treating agent solution to the following cleaning stage is diminished and some of the composition is built up because of disagreement between the composition of the reagent replenished and the process conditions of the chemical conversion treatment line, the balance between consumption and supply of treating agent components is disturbed. By way of illustration, there are cases in which sodium ions and nitrate ions are built up to abnormal levels, with the result that such chemical conversion defects as yellow rust and thin spots may take place. Therefore, if nitrous acid could be used in lieu of sodium nitrite as a chemical conversion accelerator, the accumulation of sodium ions would be successfully avoided. Actually, however, nitrous acid is so labile that it cannot exist under ordinary conditions and, therefore, cannot be utilized as an accelerator.
Moreover, in the above chemical conversion line, carry-overs of the treating agent solution are washed off with a large quantity of water and discharged out of the line and this entails troubles in the conservation of water resources and environment. To overcome these disadvantages, there has been developed a system such that the aqueous cleaning stage is constituted as a multi-stage system and the washings overflowing the downstream cleaning stage is recycled as cleaning water to the upstream stage to thereby economize the cleaning water or a system such that the washings discharged from the chemical conversion line are recovered in a closed system including a reverse osmosis stage or an evaporation stage and reused as the reagent solution to be fed to the chemical conversion bath and/or as cleaning water. In these systems, however, if an aqueous solution of sodium nitrite is fed as said accelerator to the zinc phosphate chemical conversion bath, the sodium ion tends to be accumulated in the treating agent and this has been a major drawback in the use of a closed system.
Previously, in JP Application 2000-141893, the inventors of the present invention proposed an aqueous zinc nitrite solution which is substantially free of sodium and sulfate ions and, as such, is of use as a metal surface chemical conversion accelerator, said solution being obtainable by the reaction of zinc nitrate with calcium nitrite and subsequent purification.
The present invention has for its object to provide a metal surface-treating method which comprises forming a zinc phosphate film compatible with the subsequent cationic electrocoating of a shaped product of metal, particularly a metal product having both an iron type metallic surface and a zinc type metallic surface, and which leads itself well to the implementation of a closed system.
The present invention, therefore, is directed to a metal surface-treating method
which comprises a chemical conversion step of dipping a substrate in an acidic aqueous zinc phosphate solution,
and uses an aqueous zinc nitrite solution as an accelerator,
said aqueous zinc nitrite solution being substantially free of calcium ion and containing 0 to 6500 ppm of sodium ion and 0 to 20 ppm of sulfate ion in case of assuming the concentration of zinc nitrite [Zn(NO2)2] in said aqueous zinc nitrite solution to be 10% by weight as NO2.
The acidic aqueous zinc phosphate solution mentioned above may contain 0.5 to 2 g/L of zinc ion, 5 to 30 g/L of phosphate ion, 0.2 to 2 g/L of manganese ion and 0.05 to 0.3 g/L as NO2 of zinc nitrite.
Further, the acidic aqueous zinc phosphate solution mentioned above may contain 0.3 to 2 g/L of nickel ion.
Furthermore, the acidic aqueous zinc phosphate solution mentioned above may contain 3 to 30 g/L of nitrate ion.
The substrate mentioned above is preferably a metal product having an iron type surface and a zinc type surface or one having an iron type surface, a zinc type surface and an aluminum type surface.